Bird also needs to change its feathers regularly, i.e moulting. It is one possibility if the bird is now in moulting. Another unnatural cause of missing tail feather would refer to losing feathers when it was captured and then damaged by human. We sometimes cannot tell the exact cause of bird without tail feathers.

Another interesting point is that this should be a Japanese Paradise Flycatcher from very dark head and breast and dark brown upperparts with no rufous tone.